1. Field of the Invention
The present invention relates to a metal gasket for sealing between a pair of confronting surfaces and, more particularly, to a multi-layer metal gasket interposed and clamped between the confronting surfaces of the cylinder block and cylinder head of, for example, the internal combustion engines.
2. Description of the Prior Art
Conventionally, the gaskets have been interposed and clamped between the confronting surfaces of the cylinder block and cylinder head of the multicylinder engine by means of the bolts. Various types of gaskets have been developed to insure a gastight fit between the confronting surfaces of the cylinder block and cylinder head.
Recent research and development on engines has been concentrated on less weight, more compactness or higher horsepower developed and the cylinder heads of aluminum alloy have prevailed for less weight. It has been well known that the aluminum alloy is light in weight, but less in rigidity. The engines of aluminum alloy display the great thermal expansion and/or contraction in accordance with the variation of temperature, which depends on the operating conditions of the engines. As a result, the heat loads owing to the stresses repeatedly exerted during the engine operation and the changes in engine processes render strains to the gaskets, which may cause cracks in the gaskets resulting in a steep reduction in the surface pressure, or the sealing stress, at the interfaces of engine parts with the troubles such as leakage of gases out of the interfaces.
Heretofore various types of metal gasket have been developed for sealing means instead of asbestos-made gaskets. The metal gaskets are in general provided therein with apertures in concentric alignment with cylinder bores which are in the cylinder block for reciprocation of pistons, the apertures being each surrounded with annular corrugations, or sealing beads. The gaskets further have other relatively small holes or perforations for clamping bolts or the like, which are also surrounded with annular corrugations, or sealing beads.
With the metal gasket being interposed and clamped or held between the cylinder block and cylinder head by means of the bolts, the sealing beads may make elastic metal-to-metal contact with the confronting surfaces, especially, at the annular areas around each of the cylinder bore apertures and perforations for coolant, oil, bolts or the like to thereby provide an increase in sealing stress at these areas while adequately sealing the cylinder bores, coolant and oil passages and bolt holes.
Well-known prior metal gaskets are the multi-layer metal gaskets as disclosed, for example, in Japanese Patent Publication Nos. 77833/1992 and 33178/1996. The metal gaskets are usually of plural metal plates comprising a pair of core plates with sealing beads arranged in an overlapped relation, and two sheets of intermediate plates interposed between the core plates. In the prior metal gaskets disclosed in the above citations, any one of the intermediate plates is partially folded back so as to have turns and flanges, while the other intermediate plate is provided for adjusting the thickness in plates.
The multi-layer metal gasket shown in the above citation No. 77833/1992 has two sheets of intermediate plates, the first one of which is partially folded back into flanges around the cylinder bore apertures, the second plate being less in thickness than that of the first plate and arranged spaced apart from the folded flange while in contact with the crests of the corrugations, or the raised surfaces of the sealing beads, of the core plates. The metal gasket as described just above is adapted to compensate the clearances between the confronting surfaces while providing the double sealing areas around the cylinder bore apertures. The core plate with which the second thinner intermediate plate is in contact may be prevented from flattering out of the sealing beads, resulting in substantially steady sealing performance.
The multi-layer metal gasket disclosed in the above-citation No. 33178/1996, in addition to the advantages in the gasket described just above, may protect equally the sealing beads of a pair of core plates by the prevention of the flattering phenomenon of the sealing beads in the core plate with which the second intermediate plate has no contact.
The intermediate plate in the metal gasket is ordinary rendered greater in thickness in annular areas adjacent to the cylinder bore apertures than in the remainder of the plate by folding back the intermediate plate at the annular areas around the apertures. On most metal gaskets the annular portions greater in thickness in the intermediate plate may compensate the clearances between the confronting surfaces, which are apt to widen at the circumferential area of the cylinder bore apertures to thereby protect the clearances between the confronting surfaces from the variations owing to the high pressure and heat of the combustion gases. Hence, the thicker portions cooperate with the corrugations, or sealing beads, to provide double-sealed areas higher in the surface pressure, or the sealing stress, around the apertures, resulting in preventing the leakage of combustion gases.
On the cylinder block made of cast aluminum alloy it has been, however, experienced that the thicker portions in the metal gasket may cause the excessive concentration of surface pressure, or sealing stress, between the confronting surfaces on the circumferential areas of the apertures, irrespective of whether the cylinder liners are fitted. The concentration of the sealing stress renders the strain greater at the annular opened ends of the cylinder bores, or at the annular cornered ends at which the cylinder walls are abutted against the flanged portions of the cylinder liners fitted in the cylinder bores. The strain brings about the reduction in engine performance.
Deformation in the cylinder bores due to the strain rises the fuel consumption rate, increases the oil consumption, causes the premature wear owing to unexpected friction occurring between the cylinder walls and piston rings, and if worst comes to worst, results in seizure owing to scarfing.
On most cylinder blocks of cast aluminum alloy lacking in the hardness at the open-deck surface, the metal gasket has been required to provide compensating means for protecting the corrugations, or sealing beads, from the flattering out or permanent set in fatigue by dispersing the surface pressure, or sealing stress, in the deck-surface for areas other than the areas adjacent to the cylinder bores. As a result, the sealing stress in the deck surface does not concentrate excessively on the specific areas adjacent to the cylinder bores, but disperses uniformly whereby the cylinder bores are protected from deformation due to strain while both sealing performance and engine performance may be improved.